The Great Purge encouraged Hook's increasing ambivalence toward Marxism. In 1939, Hook formed the Committee for Cultural Freedom, a short-lived organization that set the stage for his postwar politics by opposing "totalitarianism" on the left and right. By the Cold War, Hook had become a prominent anti-Communist, although he continued to consider himself both a democratic socialist and a secular humanist throughout his life. He was, therefore, an anti-Communist socialist. In 1973 he was one of the signers of the Humanist Manifesto II.[6]

Hook married Carrie Katz in 1924, with whom he had one son, John Bertrand Hook. They separated in 1933.[13][14] Katz had studied at the Rand School in the early 1920s. There, she studied under Scott Nearing and came to write a chapter in his book The Law of Social Revolution entitled "The Russian Revolution of 1917" (1926). Friends from the Rand School included Nerma Berman Oggins, wife of Cy Oggins. She was a Communist Party member who was a "Fosterite" (i.e., she supported William Z. Foster amidst Party factionalism in the last 1920s). She went on to work at the Labor Defense Council.[15]

Sidney Hook's book, The Hero in History, was a noticeable event in the studies devoted to the role of the hero, the Great Man in history and the influence of people of significant accomplishments.

Hook opposed all forms of determinism and argued, as William James did, that humans play a creative role in constructing the social world and to transforming the natural environment. Neither humanity nor its universe is determined or finished. For Hook this conviction was crucial. He argues that, when a society is at the crossroads of choosing the direction of further development, an individual can turn to play a dramatic role and even become an independent power on whom the choice of the historical pathway depends.[17]

In the book, Hook provides a great number of examples of the influence of great people, and the examples are mostly associated with various crucial moments in history, such as revolutions and crises. Some scholars have critically responded because, one of them claims

he does not take into account that an individual's greatest influence can be revealed not so much in the period of the old regime's collapse, but in the formation period of a new one (according to our model it is the fourth phase – see below). Besides, he does not make clear the situation when alternatives appear either as the result of a crisis or as the result of Great Man's plan or intention without manifested crisis.[18]

Hook introduced a division of historic personalities and especially leaders into the theories of the eventful man and the event-making man, depending on their influences on the historical process.[19] For example, he considers Lenin as having been an event-making man, in the perspective of certain important circumstance such as his changing the developmental direction not only of Russia but also of the whole world in the 20th century.

Hook attached great importance to accidents and contingencies in history[20] thus opposing, among others, Herbert Fisher,[21] who made attempts to present history as ‘waves’ of emergencies following after another

^Edward S. Shapiro, ed. (1995). Letters of Sidney Hook: democracy, communism, and the cold war. M.E. Sharpe. p. 2. ISBN9781563244872. This faith in rationality emerged early in Hook's life. Even before he was a teenager he proclaimed himself to be an agnostic. It was simply irrational, he declared, to believe in the existence of a merciful and powerful God in the face of widespread human misery. Only the pleadings of his parents that he not embarrass them in front of relatives and friends convinced Hook to participate in a Bar Mitzvah ceremony on his thirteenth birthday. People frequently asked him in his later years what he would say if he discovered after death that God really existed. He answered that he would simply state, "God, you never gave me enough evidence."|accessdate= requires |url= (help)